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major funding for earth revealed was provided by the annenberg/cpb project. this ancient meandering river
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provided sustenance for one of the earliest civilizations on earth. the culture which thrived along its fertile valleys forever changed western civilization. "egypt," said herodotus, "is the gift of the nile." puzzled by the source of all this water, many early philosophers theorized that the waters of the nile, as well as all other rivers, originated from a system of boundless underground fountains. then in the 17th century, french scientist pierre perrault conducted a simple experiment that would yield a startling discovery. perrault reasoned that rivers transport snow and rain from the land to the oceans. to test this hypothesis, he compared rainfall with the flow, or discharge, of a river.
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he first measured the amount of water flowing annually in the river seine in france. then he calculated rainfall for the upstream drainage basin surrounding the river. perrault found to his surprise that rainfall was six times as large as the flow of the river. so early speculation about rivers actually addressed the wrong question. the problem was not... where does river water come from? but...where does all the excess rainfall go? only about 1/4 of earth's annual precipitation flows in rivers. the rest seeps underground to become ground water or is stored as glacial ice or soil moisture or is returned to the atmosphere by evaporation and growing plants. rivers are among the most common land forms on earth. although they appear to vary a great deal in their behavior and characteristics, careful study has shown that all rivers have a great deal in common. the impact of rivers on the landscape
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is often spectacular. they can gouge out deep canyons, create gentle valleys with verdant meadows, or build enormous deltas. in creating these diverse landscapes, all rivers function in the same manner. they erode, transport, and deposit sediment. these processes enable rivers to continuously reshape the surrounding land. one of the most important factors influencing the geologic impact of a river is the velocity of its water. a swiftly flowing river erodes and transports more sediment than a slow river. velocity generally increases with the slope of the river, but channel shape also plays a role. if a channel has a nearly perfect semicircular cross-section, the frictional resistance is the minimum, so the water loses very little energy
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flowing over the channel. if it's a wide, flat channel, a fairly shallow river, there's a greater surface area along the banks and the bottom, and that slows the stream down, too. the texture of a stream bed also influences stream velocity. roughness is a function of the materials over which it flows, so if it's flowing over gravel and boulders, there's more resistance to the flow. that slows the river. if it's flowing over muds and clays, like along the lower mississippi river, there's less resistance, and it flows a little faster. the velocity of a river also tends to increase if the amount of water in the river channel increases. the quantity of water moving through a river is called its discharge. the discharge of a river is how much water it's actually carrying. we usually measure this
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as a volume per unit time. the united states commonly says cubic feet per second. most of the world uses cubic meters per second moving down the channel. discharge increases from the head of the stream to the stream mouth as the drainage basin increases. there's simply a larger area to contribute discharge, to contribute flow to the streams. the primary way that a river functions geologically is to transport not just water, but sediment, down slope and toward the oceans. the faster a river flows, the more efficient this process becomes, so geologists are acutely interested in flow velocity. when the flow velocity of a stream is relatively high, the energy of the moving water is converted into processes that lift chunks of bedrock or sedimentary particles from the bottom and carry them downstream. this is known as erosion. there are three different erosional processes
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that operate in rivers. the first is hydraulic action. the turbulence of a rapidly flowing stream applies vertical forces that lift sedimentary grains off of the bottom. the flowing current also pushes against these particles and carries them downstream. if you've ever waded across a river and felt the sandy bottom moving beneath your feet, you've experienced hydraulic action. the faster the river flows, the greater the turbulence, and the swirling flow of a very rapid stream can even wrench chunks of fractured bedrock off of the channel bottom. the rapid current of a sediment-laden river can also generate a sand-blasting effect which can scour its way down through sediment or even solid rock. in this process, called abrasion, the energy of the moving water is converted into collisions between sedimentary grains and the bedrock of the channel bottom. abrasion not only smooths river cobbles into rounded shapes, it can also wear away the bedrock many times faster than hydraulic action alone.
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running water also, to some degree, dissolves any type of rock or mineral. this process of erosion, called dissolution, is controlled in part by the mineral composition of the bedrock. for example, a river bed made of limestone will dissolve more rapidly than one made of granite. the rate of erosion by dissolution is also controlled by temperature, the acidity of the water, and by flow velocity. erosion in its various forms is only one way rivers interact with the sediment and bedrock of earth's crust. once this material's picked up and put into motion, it becomes part of the river's flow and is transported downstream by one of several processes of sediment transport. the shape, size, and composition of sediment influence how the sediment will be carried along in the stream and where it will be deposited. larger particles stay near the bed of the stream and is transported by rolling or bouncing
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or skidding along the bottom. this is called bed load. when material's moved as bed load in a stream, exactly how it moves is largely a function of size. larger particles-- gravel, cobble, or boulder size-- stay in contact with the bed virtually all the time, except in extreme discharge events where the velocities are very high. these particles move by rolling or by being pushed or by sliding along the bottom. this is the traction load of the stream, continuously in contact with the bed. smaller particles in transport as bed load-- sand grains, for example-- stay close to the bed, but aren't in contact continuously. these particles actually move along in a series of jumps-- hopping up into the flow, being pulled forward by the discharge, hitting bottom, bouncing up again, or ejecting another particle from the bed which jumps up into the flow.
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this style of bed-load transport is called saltation. although considerable amounts of sediment are transported as bed load, most of a stream's sediment is typically carried in suspension and in solution. suspended load includes material like silts or clays. it's light enough to be swept along in the current without touching bottom. dissolved load is invisible. it is the ever-present soluble material which results from chemical weathering of the rocks along the channel. because precipitation varies seasonally, as well as from year to year, the discharge and velocity of a stream also fluctuates. as the river slows down, the turbulence of the moving water begins to subside, and the amount of energy available to erode and transport sediment decreases abruptly. much of the sediment no longer remains in motion, and is deposited, instead. the sediment is usually deposited in the river channel itself
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in a series of piles, called bars. most river bars are ridges made of sand and gravel that are covered with small migrating ripples. in fact, the bars themselves are actually large ripples that migrate downstream during sporadic cycles of erosion and deposition. bars are especially common in braided streams which form where sediment-choked rivers flow across broad, easily-eroded slopes. bars also commonly occur in meandering rivers. a meandering river typically wanders across wide valleys and lowlands in a series of s-shaped curves. erosion and deposition occur continuously, side by side, along the banks of meandering rivers. low velocity on the inside of a meander curve results in the deposition of point bars. on the outside of a curve,
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where velocity is high and erosion normally takes place, cut banks form. because of this erosion and deposition, both the sizes and positions of meanders continuously change. we're not altogether clear why meandering occurs, why it's such common phenomenon. it probably has to do with the equalization of energy distribution as the flow moves down valley. by meandering, the amount of work done by a stream in some unit of discharge is more or less constant, and that seems to be a principle of nature-- to try to equalize the amount of work and minimize the amount of work at the same time it's being done. the mississippi is a prime example of a meandering river. as the crow flies, the distance between new orleans and memphis is about 550 kilometers. by boat, it's over 1,000. meandering rivers are associated with one of the world's
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most significant geological hazards-- flooding. floods are absolutely a natural part of the river's cycle. in fact, flooding, that is to say, overbank discharge, is common enough that it shouldn't surprise anyone. geologists have looked at this pretty carefully over the past few years. we have records which indicate most streams overtop their banks about every two and a half years. that's nothing unusual at all. anyone who is surprised by flooding are the ones that are not paying attention. shifting meanders and repeated flooding along rivers produce broad flatlands called flood plains. when river goes into flood, the water level in the river channel rises until water spills over the river bank, drowning the adjacent landscape and giving the flood plain its name. human population centers have historically been closely linked to the flood plains of major rivers like the tigris and euphrates in ancient mesopotamia,
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the yangtze and huang ho in china, and the nile in egypt. flood plains are good places to grow crops because as each flood inundates the plain, it carries with it a muddy sediment rich in organic matter and nutrients. the sediment is deposited in flat layers atop the flood plain and is naturally irrigated by the flood waters. but life on the flood plain is a double-edged sword. the agricultural benefits of those periodic floods are offset by damage to homes and cities, and, in some cases, to the people who inhabit them. the edges of flood plains are marked by levees- ridges of sediment left atop river banks by floods. once formed, levees serve as natural barriers confining rivers during periods of ordinary flow. they may even protect low-lying areas from flooding if the level of a river isn't too high.
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for this reason, artificial levees designed to contain a river during flood stages are often built. but artificial levees can themselves create problems. by confining the river to a narrow channel, levees accelerate the build-up of sediment, raising the river bed higher and higher. and levees can provide a false sense of security. if a river overtops its levees to flood the surrounding land, the levees can actually prolong flooding by preventing water from draining back into the river. most people don't appreciate the fact that the flood plain is a part of the stream itself. the flood plain is where rivers store discharge during periods of high flows and also places where rivers store sediment. when we move on to the flood plain, we're moving on to the river. it's not very different from being in the channel.
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it's just that the river doesn't use it all the time. one way to reduce floods is by constructing dams through which a river's discharge can be regulated. but while dams solve some problems, they can create others. all man-made structures in a river valley have an effect upon the stream, the most profound effect caused by dams. a dam creates an artificial base level-- sea level, for example, that causes the stream to deposit all of the load that it's been moving. the quiet water of the lake doesn't allow sediment to move, so it's dumped at the upper end of the reservoir. the water which comes through the spillways of the dam is now without the sediment that it's been transporting and will go about eroding new sediment to replace that which has been lost. a river replaces this sediment by eroding the river channel
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downstream from the damn. sometimes this erosion can be severe. the basis of river dynamics is a state of balance between erosion, transportation, and deposition. this is what every body of running water naturally seeks from its headwaters to its mouth. water literally has the power to move mountains in its quest for equilibrium. the stream will always try to exist in a state of equilibrium between the load it's carrying and the discharge that it has. if the load decreases, the stream has excess energy which will usually be used to erode the bed and banks. if the load increases, the stream won't be able to handle it, so some of it will be deposited. one place where human activity has come into conflict with a great river seeking to maintain its equilibrium,
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is the mississippi. stretching almost 4,000 kilometers, the mississippi drains approximately 42% of the united states. it is a sediment-laden river, shifting an estimated 516 million tons per year from its headwaters in minnesota all the way to the gulf of mexico. along the great length of this river, the process of deposition sometimes causes serious problems. if bars build up in important areas of navigation, they can disrupt shipping and regional commerce. in the industrial corridor between new orleans and baton rouge, lies one of north america's most important navigational routes. in order to keep the river open to the many ocean-going vessels which use it year-round, the united states army corp of engineers
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must continually grapple with the forces of nature. one frequent trouble spot lies just south of baton rouge in a stretch of the river called red eye crossing. here the river tends to deposit sediment, threatening to close the channel to deep-water ships. a detailed study of red eye crossing is currently underway at the army corps' waterways experiment station, or wes, in vicksburg, mississippi. tom pokrefke is chief of the river engineering branch and heads the red eye investigation. the problem that we're studying on the red eye crossing is the crossing itself. when you go from a low water situation to a high water situation, it tends to fill with sediment. where you go from high water to low water, there's not enough energy in the water
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to clear that crossing out and maintain the channel deep enough for ship-type navigation in that part of the river. basically, the red eye crossing area has been kept open in the past using dredging. when the water filled going from a low water situation to a high water situation, the engineers dredged the channel to make sure it was deep enough. the army corps of engineers would like to minimize the amount of dredging necessary to keep the channel clear. at the core of their study is a scale model of the river. graded, crushed coal is used to represent the bed material. by studying how the coal moves as water is discharged through the model, the corps' hope hope to better understand the red eye crossing problem and come up with solutions. each experiment with the model has a known fixed amount of water discharge. water fills the channel
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and is allowed to run for a measured length of time. almost immediately, the bed load begins to move. confetti thrown on the water surface during the experiment, clearly indicates the water's flow patterns through the crossing. the white beads are another indicator of how the model is performing. the beads build up where expected, at the place where, in nature itself, the point bar exists. this indicates the model simulates nature accurately. plus four. periodically, the model is drained, and its sediment is carefully mapped. this detailed mapping gives engineers a better understanding of sedimentation processes in the river. using data from the movable bed model and from the field,
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wes engineers have completed a computer model which calculates the movement of sediment through red eye crossing. it mimics the sediment's behavior through time as water discharge and velocities change. the light blue area is deep water. the dark blue-- shallow water, which corresponds to sediment build-up. as the model goes from low discharge levels to high discharge levels and back to low again, the dark blue area grows in size indicating that sediment is moving in causing the deep water channel to narrow. potential solutions to the problem at red eye crossing are tested on the computer. the construction of walls or dikes within the channel is factored into the program. according to the computer, dikes help to eliminate the point bars.
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eventually, dikes will be built and tested on the physical model to check their effects on sediment transport. the way dikes function as far as opening a channel and making it wide enough and deep enough, is they actually take the channel that has a relatively wide width from top bank to top bank, and it contracts it normally on one side and makes it a little bit narrower. what that does is mother nature and the river itself says, i need to have so much area available to me. when you pinch in the sides, the only thing that can happen is the bed scour. you want to make sure the bed scour's enough, that the channel is wide enough and deep enough year around, be it high water or low water. also, you don't want to pinch it down too much that all o of a sudn the velocities start getting high going through that dike field, and then it becomes a problem to navigation, also.
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dikes seem to be the most effective way to reduce the need for dredging and keep the channel open. but before they are installed at red eye crossing, the engineers want to determine how the dikes will affect the people who actually use the channel-- the ship and towboat pilots. the pilots of the mississippi river have been part of the region's lore for many years. they know the river bette than anyone else possibly could. guiding a ship or boat down the mississippi means far more than simply memorizing a route from point "a" to point "b." for the mighty mississippi is a dynamic system, always shifting and churning. as mark twain so knowingly wrote in life on the mississippi-- "two things seem pretty apparent to me. "one was, that in order to be a pilot, "a man had to learn more "than any man ought to be allowed to know,
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"and the other was that he must learn it "all over again in a different way every 24 hours." the wes facility includes a ship/tow simulator which functions much like a flight simulator. here, the navigating instrumentation can be configured for either a towboat or a ship. the screen is an accurate representation of the view from the pilot house. many pilots are brought to the waterways experiment station during the study. each spends a week repeatedly steering up and down the computer-simulated course of red eye crossing. the proposed dikes are factored into the simulation. every run down the crossing is different. the computer changes many parameters, such as river discharge, the number and placement of passing ships, and channel depth.
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as the pilot wends his way down red eye, the computer records the exact course of each run, the time from start to finish, and whether or not there were any collisions or other safety problems along the way. although dikes seem to be a promising solution, the army corp study indicates that they might create some navigational problems. so the investigation continues. and the mighty river remains unshackled as it flows through red eye crossing. the power of running water extends far beyond the mississippi. indeed, it is the dominant force shaping earth's landscape. the combined discharge of all the rivers on earth is only 1/10,000th of one percent of all of the water on this planet, but few geologic processes have exerted a greater influence on human history and civilization. many of the world's great cities
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were first established as riverside settlements, and throughout their history these cities have depended on the river for food, a water supply, and an avenue of transport and trade. but like all natural systems, rivers undergo relatively rare but extreme events. river flooding is a threat to nearly every nation on earth. in the united states, floods exact the greatest toll of any geologic hazard, causing billions of dollars in property damage and killing about 100 people every year. and this loss is modest when compared to the destruction in countries with primitive flood control systems, or the devastation in preindustrial societies which were visited by floods without warning. like most natural systems, rivers change and evolve through time in response to a variety of geologic factors that are themselves changing. factors such as regional climate, hill slope, tectonic activity, vegetation, and the bedrock composition
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of the earth's crust. so the behavior of rivers is controlled by physical laws and geologic processes that can be observed and understood. rivers do much more than drain water from the land and carry sediment to the sea. the evolution of a river exerts a powerful influence on the surface of the earth. in fact, much of the continental landscape, especially those areas where people live, was formed by the power of running water. captioning performed by the national captioning institute, inc. captions copyright 1991 the corporation for community college television
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major funding for earth revealed was provided by the annenberg/cpb project.
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major funding for earth revealed was provided by... captioning made possible by southern california consortium a close look at the earth's intricate system of running water
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is a close look at the evolution of earth's landscape. as they continue to shape the land around us, rivers and streams leave behind evidence of their enduring power. unlike earthquakes and volcanoes, which can cause sudden change, running water works slowly, almost imperceptibly, in shaping earth's landscape. we usually think of the grand canyon in terms of its rocks and the fascinating story that they contain, one that spans almost half of earth history. but there's more to the grand canyon than rocks. the canyon itself is a geologically active feature, a changing and evolving land form that's a monument to the power of running water. the colorado river carved this enormous valley over the last nine million years. in fact, the river carries
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about half a million tons of sediment past any point in the canyon every day. no wonder this great river has been described as too thin to walk on, but too thick to drink. rivers like the colorado, are powerful geologic agents that disrupt and reshape the surface of continents. the energy of running water in a river channel is transformed into processes that erode rock and sediment from the bottom of the channel and carry it downstream. as the river deepens its channel, the sides of the valley steepen and grow unstable. eventually mass wasting processes are triggered, causing these slopes to fail. this delivers even more sediment to the river and widens the river valley even further. in order to understand the influence of running water on the earth's surface, we're going to look at a variety of rivers and also at different land forms in various stages of their development. in doing so, we'll explore the connections between the geologic process of running water and the evolution of the surface of the earth.
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the connection between a river and its deep, wide valley is not an obvious one. at one time, valleys were thought to have formed independently of the rivers which flow through them. today geologists are well aware that valleys usually form by the down-cutting of running water combined with the mass wasting of slopes. as a river cuts its channel deeper, it carries away sediment fed to it from surrounding hillsides. there are limits to how deeply a stream can erode its valley. those limits come of several kinds, generally referred to as base level. the ultimate base level, or grand base level, is sea level. stream don't degrade their valleys below the level of the sea. we don't find great canyons arcing down to the ocean filled by water which flows in from the sea. the stream, as it approaches sea level, loses velocity and therefore loses ability to erode.
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modern concepts of landscape evolution began with an american geomorphologist, william morris davis. he believed that rivers and streams gradually wear down rugged mountain slopes to form plains. he classified landscapes by their maturity and used the terms "youthful", "mature", and "old age" to categorize their stages of development. davis' work, done mostly in the appalachian mountains, conceived of landscapes as going through a distinct series of stages that began with an uplift of the area, supplying streams with potential energy which they could then use to carve their valleys. during the earliest stages after uplift, streams predominately cut downward, incising their valleys, and creating a steep-sided landscape. davis referred to this as "youth." as this process continues, the streams gradually expend some of their energy
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carving from side to side, wandering back and forth across the valleys. the valleys then cease to be v-shaped, become somewhat more flat-floored, and eventually all of the original upland surface is consumed by erosion from tributaries, so that all of the landscape is now in hill slope. this is what davis considered to be the stage of maturity. and finally, as the stream works down, it reaches near base level, or the limit to which it can erode downward and then does very little vertical erosion, expending almost all of its energy eroding from side to side. and as it does so, these valley bottoms become more and more broad, the tributary streams erode their slopes, eventually creating flood plains of their own, and, finally, virtually all of the material that was uplifted is destroyed and brought to an equal level, which davis referred to as a peneplain, as the ultimate stage in the cycle of erosion which he thought ended in "old age."
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in order to make his model easier for geologists to apply, davis described uplift and erosion as events occurring separately in time. but he knew, in fact, these processes occur simultaneously. landscapes form by a continuous interplay of tectonic activity and erosion. other crucial elements also influence the shape of the land, including rock type, rock structure, and climate. solid rock, for example, can hold up a much steeper cliff than sand or clay. and when structures, such as folds or faults, appear at the earth's surface, shapes adjust accordingly. the slope of the grand canyon is a result of many of these factors. the rocks are layered sedimentary strata with contrasting resistance to weathering. this naturally results in slopes with differing steepness
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and rates of erosion. in addition, the arid climate and relative lack of vegetation contribute to the sharp, angular features of the canyon walls. ultimately, rock type and rock structure also affect river drainage patterns. by studying different stream patterns, geologists can infer a great deal about the nature of the underlying rock. if we have homogeneous rocks or flat-lying sedimentary rocks, streams typically form what's called a dendritic pattern that looks much like the branching of a tree. where the rocks aren't homogeneous or there are definite structures in the rocks, other patterns occur. for example, if we have intersecting fractures or faults, streams commonly follow those intersecting patterns. they make sharp bends at acute, or even right angles, forming a different kind of stream pattern. where we have alternating layers of strong and weak rocks, the streams more usually branch out as a trellis pattern,
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reflecting the weaker valley rocks and the stronger hill-forming rocks. the drainage patterns of streams expand and grow more intricate as the land erodes away. even after streams and rivers wear a landscape flat, it's possible for erosion to become active again if the landscape is uplifted or if the regional base level drops. geologists call this renewal of stream erosion "rejuvenation." and it produces certain characteristic land forms such as stream terraces. stream terraces are found frequently in areas of very wide valleys where one of several things has happened. one common way in which terraces form is because of rejuvenation, where there has been uplifting of the area
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or a down-dropping of the base level, so that a stream which was formerly meandering with big, wide sweeping turns, with a wide flood plain, cuts into its own flood plain, leaving the flood plain elevated on either side of the river as a terrace. it is no longer an active flood plain, but an elevated surface above the river. but terraces can also indicate other types of regional change. for example, if the climate grows drier, a river will shrink in size. rather than carrying away sediment from surrounding slopes, the shrinking stream will eat into its own sedimentary deposits created in wetter times. gradually terraces form. another land form that can be produced as a result of rejuvenation and uplift is an incised meander. the key factor distinguishing incised meanders from normal meanders is that they are cut well below the level
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of a river's former flood plain. incised meanders result from down-cutting along the thalweg-- or deepest part of a river's channel. the down-cutting is so rapid the river maintains a meandering pattern while deepening its valley. river valleys form a significant part of earth's landscape, but they aren't the only land form created by running water. all streams and rivers come to an end. most ultimately flow into the ocean or another large body of water, such as a lake. due to the sudden loss in velocity at the mouth of a river, most of its sediment is deposited, forming a delta. deltas, of course, form at the mouth of the river where they enter a large lake or the ocean. the gradient or the slope of the river
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is very gentle. when it hits the water, there's no gradient. consequently, the sediment begins to settle out immediately. in so doing, it dams its channel. the river tends to branch into a series of distributaries. from time to time, certain branches load up with more sediment than others, so the main flow of the river may shift from one locality to another over a long period of time. distributaries play a vital role in building and enlarging a delta, intermittently supplying new sediment to all parts of the delta's shore. the mississippi river has built one of the largest deltas in the world. nearly 40,000 square kilometers of land have been added to the state of louisiana due to the astonishing power of the mississippi river and its enormous amount of sediment. one million tons of silt, sand, and clay are added to the mississippi delta each day,
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giving the river its nickname-- the big muddy. the mississippi could not have created this much land if it had stayed in one channel. the southern part of the river has changed course many times over an area some 300 kilometers wide. the key to these changes is the river's natural tendency to follow the path of least resistance, which is almost always the shortest route to the sea. the mississippi follows a single channel until gradually its channel fills with sediment. at that point, the river easily overtops its banks during periods of high discharge. when that happens, it is free to find a more direct route to the gulf, until, of course, the lengthy cycle begins again. this cyclical shifting of the mississippi has resulted in an ongoing battle to control the forces of nature.
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along most of its lower course, levees have been built to confine the river to its present channel. cities and ports have grown along the mississippi, and it has gradually become one of the world's most important economic waterways. if the mississippi were allowed to change course from its modern channel, major ports built along its shores would be left dry. elsewhere, farms and towns in the path of the new riverbed could be washed away. so the u.s. army corps of engineers has engaged a team of scientists and engineers to hold the river to its present channel. how long the corps can keep the river where it is is really just a matter of money. one of the things about engineering is that that you can do almost anything, given the money.
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we can basically keep the river where it is. we may have control structures up and down the river, because the river will try to change course to find the shortest distance to the gulf of mexico. it may be not this flood, but maybe the next where a levee might break or else a structure might be flanked or something like that, where the river will try to change its course again. but the corps realized it could not really let this happen. the economies of baton rouge and new orleans depend on the river for its fresh water, for its commerce, its transportation. industries all up and down the river use the fresh water in their processing. in its continuing search for the shortest route to the sea, the mississippi has found a comrade. at one time a mere trickle compared to the mississippi, the atchafalaya is now a mighty predator,
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the mississippi-- a willing prey. the fight to control the mississippi has escalated from a battle into a war. approximately 150 miles north of new orleans, these two rivers have come perilously close together, linked by an abandoned loop of the mississippi called old river. the atchafalaya offers the mississippi a route to the gulf that is 175 miles shorter than its present course. the corps of engineers realized there was a potential problem with the atchafalaya capturing the mississippi back in the 1950s. a gentleman by the name of fisk, who was a geologist, did a report for the corps of engineers and the mississippi river commission in which he studied old delta systems of the mississippi and old diversions of the mississippi and compared those to what was happening on the atchafalaya and the mississippi. and he theorized that
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the atchafalaya and the mississippi were in an intermediate stage of capture and that, if something was not done by about the 1970s, about 1975, we would reach a critical stage of capture in which the mississippi would no longer be able to carry any more flow because it was filled with sediment, and the flows would go down the atchafalaya. he said that this would happen when about 40% of the flow was going down the atchafalaya from the mississippi. in 1954, as a result of the army corps report, the united states congress authorized the old river control project. essentially, this funded the construction of a series of control structures and channels all situated in the old river area. under the plan, the flow of water and sediment between the mississippi and the atchafalaya was to remain at its then current rate,
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a 70/30 split. 30% of the combined discharge of the mississippi and red rivers was to flow into the atchafalaya, while the remaining 70% would be kept within the mississippi itself. first the old river channel was damned. this meant that the only natural connection between the atchafalaya and mississippi was closed. since the corps didn't want to disrupt boat traffic between the two rivers, a navigational lock was built on old river at a cost of $15 million. to enforce the mandated 70/30 flow rate, construction began on two control structures at a combined cost of $15 million. these structures were completed and operational by 1963. the low sill structure is 566 feet wide
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and has 11 gates which allow the corps to control the flow rate. the low sill sits on a man-made outflow channel connecting the mississippi to the atchafalaya. on the flood plain next to the low sill structure sits an emergency facility-- the overbank structure, built to assist the low sill during major floods. well over a half-mile long, it has 73 gates. for a time, this elaborate and costly system managed to keep the mississippi in place, but in 1973, 10 years after the system went on-line, the corps' efforts were tested to their limits. well, 1973 we had a flood that happened to be the second greatest flood man has observed since he kept records on the mississippi,
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the greatest being the 1927 flood which resulted in the mississippi river and tributary system and all the structures that you see today. we had to open up several structures between old river and new orleans to alleviate flood waters between here and new orleans so that new orleans wouldn't go under water. what happened is that the river decided that it basically wanted to continue going down the atchafalaya. the thalweg of the mississippi moved almost right into the entrance to the low sill structure and basically took out a wing wall. the forces were that powerful. it also undermined the foundation of the low sill structure. the water, in addition to going around it, did go underneath the structure. it was only because the structure's pile found it, and that the piles were very, very deep,
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that the structure remained standing. we basically just had to open the gates and let the river go because they were so afraid to lose the structure. although emergency repair work to strengthen the low sill structure began immediately, it was obvious that more control was needed. at an additional cost of almost $300 million, the army corps proceeded with the construction of another control structure and accompanying channel. we learned that we would basically not be able to control the flows unless we did construct another structure. the low sill was just too damaged. the corps was authorized in the late seventies to build the auxiliary structure. the auxiliary structure, which was completed in 1986, basically is what its name says-- it's an auxiliary.
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it serves to complement the low sill structure, allows us to get better control of the flows so that in the event we have a 1973 flood, which was just a tremendous amount of water, that we wouldn't get ourselves in the situation that we did where we almost lost the river. the 1973 flood demonstrated how suddenly the river's conditions could change. the corps realized it had to more closely monitor the 70/30 ratio of flow between the mississippi and the atchafalaya. until 1973, the corps only looked at the the average annual flow. as a result of the flood, the flow rate is currently monitored each and every day. it is the new orleans district office of the army corps of engineers which oversees the daily operation of the control structures. the flow data from the structures
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is reviewed here, and river conditions are closely monitored. based on these data, major decisions are made about which gates are to be opened or closed. a small device is lowered into the water to measure velocity. these measurements are made along the river to calculate discharge. using this information, flow predictions are made, and the gates are raised or lowered accordingly. but the amount of water flow is not the only factor to be considered in keeping the mississippi in place. although the atchafalaya takes water from the mississippi, it leaves most of the sediment behind. in response, the atchafalaya scours its own channel, acquiring enough new sediment to restore its equilibrium.
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the effect of the scouring also deepens the atchafalaya's bed, providing an even steeper route for the mississippi. the effect on the mississippi of the bed load remaining behind is a buildup of sediment in its channel. so, sediment flow, especially bed load, as well as water flow, must be kept in check. with this in mind, the auxiliary structure was strategically placed. one of the goals of placing the auxiliary structure where it is is to increase the sediments being diverted from the mississippi to the atchafalaya. one of the lessons that we learned was that we weren't diverting the same proportion of sediments through the low sill structure as we were water. the mississippi river was continuing to show evidence that it wanted to fill up, and the atchafalaya was continuing to scour.
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we felt if we increased the sediments being diverted here at old river, we would actually try to stabilize the mississippi river and the atchafalaya river, at least slow the trend in the atchafalaya. so we located the auxiliary structure on the inside of a bend where there's actually more sediments. we angled it so we'd get the sediments moving along the river bottom. these sediments would go into the inflow channel of the auxiliary structure, through the structure, and on down to the atchafalaya. life around old river is generally peaceful now. even more important, the world below old river carries on normally. many here are unaware of their upstream fortress. it would take an extraordinary amount of water to test these structures and the will of the army corps of engineers,
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but rivers are capable of extraordinary things. our ability to control nature, particularly our ability to control rivers is limited. we may be successful for a short period of time-- a year, 5 years, 10, maybe even 25 years, but there's always a larger flood out there. there's always a bigger windstorm or higher waves than the ones we've encountered before. when we place ourselves and our lives in the paths of these processes, we can expect to see the adverse effects. since its beginnings, civilization has flocked to the riverside, and there has always been a price to pay as a result. but even with their power to destroy, rivers have given back. they have cradled the life in and around their banks and carved out landscapes which are legacies to their power and might.
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today as always, running water is one of the most significant sculptors of earth's terrain. its effects are virtually everywhere, even in places which appear to be dominated by other geologic forces. indeed, the land forms running water creates and leaves behind are an enduring testament to its power. the sequence of events that takes place in the evolution of landscapes is not completely understood. this is because the processes that shape the land's surface operate very slowly on a human time scale. but there's no doubt that runng water plays a significant role. land forms that have been shaped by running water are found in nearly every terrestrial environment on earth. they're even abundant in deserts, where sudden rainstorms and flash floods can produce more geomorphic change in a few hours than years of desert winds. but there would be no running water without slopes. land slopes are both created and maintained
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by tectonic activity. indeed, the shape of much of the earth's surface is the result of a constant competition between tectonic forces and the destructive effects of running water. nowhere is this duel between tectonism and running water easier to appreciate than here at the grand canyon, where the colorado river continues to sustain the evolution of one of the most beautiful and distinctive landscapes on earth. captioning performed by the national captioning institute, inc. captions copyright 1991 the corporation for community college television
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Democracy Now Special
LINKTV October 31, 2012 9:00am-10:00am PDT

Series/Special. Special edition of Democracy Now!

TOPIC FREQUENCY Mississippi 17, Davis 5, United States 4, Colorado 3, Nile 2, Perrault 2, The Sea 2, Us 2, Mexico 2, The Mississippi 2, Riverside 2, New Orleans 2, Tom Pokrefke 1, Fisk 1, Bette 1, Delta 1, Seine 1, Herodotus 1, Euphrates 1, Huang 1
Network LINKTV
Duration 01:00:00
Rating PG
Scanned in San Francisco, CA, USA
Source Comcast Cable
Tuner Channel 89 (615 MHz)
Video Codec mpeg2video
Audio Cocec ac3
Pixel width 544
Pixel height 480
Sponsor Internet Archive
Audio/Visual sound, color

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